1. Field of the Invention
The present invention relates to a wire clamper used in a wire bonding apparatus.
2. Prior Art
The Japanese Patent Application Publication (Kokoku) No. 61-2294 discloses one example of the conventional wire clamper.
The clamper includes a movable arm and a fixed arm, and the movable arm is urged toward the fixed arm by a spring. The movable arm is opened and closed by a solenoid. When the solenoid is actuated, the projecting rod of the solenoid pushes the movable arm overcoming the driving force of the spring, and as a result, the movable arm is opened. When the solenoid is turned off, the movable arm is closed by the spring so that the movable arm is closed and clamps a bonding wire between the movable and fixed arms.
In this prior art, the clamper arms are operated via the solenoid provided on the clamper. Accordingly, the overall weight of the clamper is large, and the response of the clamper tends to be slow. Meanwhile, the load applied onto the bonding wire (i.e., the clamping load) to hold the bonding wire is provided by a spring force of approximately 40 to 50 gr. Thus, when a bonding head upon which the clamper is installed is moved fast, the arms of the clamper tends to rattle, causing the clamping load to be unstable.
In addition, the amount of the clamping load depends on the spring. As a result, when the clamping load is to be changed, it is necessary to replace the spring. It is, however, difficult to obtain a desired clamping load by changing the spring, and considerable time is required for the adjustment of the load.
The inventor of the present patent application previously filed a patent application for a device that solves the problems as described above (Serial No. 08/031,237 based upon Japanese Patent Application No. 4-87454). In this device, the clamper has a pair of open/close arms so as to hold a bonding wire when the arms are closed, and at least one of the arms is opened and closed by the electrostrictive strain effect or magnetstrictive strain effect of a piezoelectric element.
Since the arms in this device are opened and closed by a piezoelectric element, the weight and response characteristics of the wire clamper can respectively be small and conspicuously high. In addition, since the clamping load is obtained from a piezoelectric element, the components for clamping the bonding wire do not rattle even when the bonding head moves fast. Also, since the clamping load can be changed by varying the voltage applied to the piezoelectric element, any desired clamping load can easily be obtained.
However, various prototypes and experiments of such a clamper made and conducted by the inventors unveiled several problems. Generally, in a wire bonding apparatus, a capillary is located beneath the wire clamper, and a heating block which heats a workpiece is located beneath the capillary. Accordingly, when the heating block is heated, the heat of the heating block causes the temperature of the wire clamper to rise by approximately 20.degree. C. When the temperature of the wire clamper rises, the clamping elements provided at the end of the arms move in the direction to close the arms. This results in an increase of the clamping load. Accordingly, when opening the wire clamper, a higher value of voltage must be applied to the piezoelectric element. In this case, it is often necessary to increase the voltages up to the permissible upper limit of the piezoelectric element. Thus, there is a danger that the piezoelectric element will not work properly.